Process for the preparation of metallic materials by compression of a magnesium or magnesium alloy powder



United States Patent 3 247 297 raocnss roa THE PfiErARA'rIoN "orMETALLIC MATERIALS BY COMPRESSIQN OF A MAGNE- SIUM 0R MAGNESIUM ALLOYPOWDER Marc Salesse, Gif sur Yvette, Jean Herenguel, Versailles,

Roger Caillat, Sevres, Jacques Boghen, Argenteuil, and

Raymond Darras, Versailles, France, assignors to Commissariat a lEnergieAtomique, Paris, France N0 Drawing. Filed .Ian. 8, 1962, Ser. No.165,035

Claims priority, application France, Mar. 3, 1961, 854,486 7 Claims.(Cl. 264-82) The present invention relates to a process for thepreparation of metallic materials by compression of a magnesium ormagnesium alloy powder.

The invention also relates to powders and materials obtained by thisprocess.

It is well known that the fields of use of metallic materials dependessentially upon their mechanical properties and their resistance tooxidation Within a given temperature range. In the case of magnesium andits alloys, a degradation of the mechanical properties of materials madefrom such metal occurs from about 350- 400 C. On the other hand,oxidation in air becomes very appreciable and rapidly accelerates whenthe temperature rises above 350-400 C., for example, the metal iscompletely unusable in dry or humid oxidising atmospheres or in anon-oxidising but humid medium.

Known processes for improving the mechanical resistance of magnesiumgenerally consist in utilising alloys or, preferably, in makingmagnesium materials from powders which undergo oxidation before, duringand/or after compaction of the powder.

Oxidation of magnesium particles is a very difficult operation, however,particularly with those fine powders which confer a high mechanicalresistance on the material. This also applies to the fine parts ofmixtures of powders of a range of particle sizes. With such finepowders, oxidation produces quite thick oxide films, if it does notcause, through uncontrollable elevation of the temperature,substantially complete oxidation which involves the risk of extending tothe entire mass. The products obtained have the following disadvantages:

(a) Insufliciently high rupture load, due to the impossibility of usingsufliciently fine powders;

'(b) Low rupture elongations in traction, both in the cold and hot, dueto excessive general or local oxidation;

(0) Irregular and dispersed properties when the heating of the powdershas not been suflicient before compaction;

(d) No improvement in the oxidation resistance.

The present invention has the object of providing an improved processfor the preparation of magnesium-base materials which avoids thesedisadvantages and permits such materials to be obtained from magnesiumor magnesium alloy powders. The materials produced have excellentmechanical properties, both in the cold and hot, and also have excellentresistance to corrosion at elevated temperatures in oxidising and humidatmospheres.

According to the invention, a process for the preparation of amagnesium-base material comprises compressing a magnesium-containingpowder before, during and/or after subjecting it to an atmospherecontaining fluorine or a fluorine compound at a temperature in the rangeof 0 to 600 C. so as to produce a compact material containing 0.1 to 15%by weight of combined fluorine.

The substance or substances which can be used, in carrying out theprocess, to incorporate the fluorine into the ultimate product, which isreferred to for brevity as fluorina-tion," can include gaseous fluorine,hydrogen fluoride, hydrofluoric acid or any other fluorinated substancecapable of readily evolving fluorine, and can if desired be diluted withan inert gas, such as argon.

The particle size range of the initial powder can be from 1 to 2000microns. The process can be applied to powders having particles of anyshape, such as spheres or dendrites, and also to powders havingparticles of very irregular shapes and sizes.

The temperature at which fluorination is carried out preferably liesbetween the ambient temperature and 600 C.

The duration of the fluorination step depends upon the composition ofthe gaseous phase used and also upon the temperature, and is generallyfrom 2 to 5 hours.

The fluorination step can be effected in various ways.

A first preferred embodiment consists in introducing the powder to befluorinated, before it is compacted into a receptacle made of suitablematerial, such as nickel or stainless steel, and placing the receptacleinside a sealed and fixed horizontal furance, made of a suitablematerial such as copper.

The apparatus is then purged with an inert gas, for example argon, andthen gaseous fluorine or a fluorine compound, diluted with an inert gasif desired, is introduced. The introduction of the gas and, if desired,the temperature of heating are so controlled that the temperature liesbetween the ambient temperature and 600 C.

Another embodiment, in which fluorination is also eflected beforecompaction, consists in using a rotary furnace containing balls. In thiscase, the powder is continuously eroded as it is fluorinated. The ballscan be of an appropriate material, such as steel covered with magnesium.

This process allows very fine particles to be obtained and lowertemperatures to be used, for a given ombined fluorine content, ascompared with the first embodiment. Also, the subsequent compaction ofthe treated powder more readily yields a material having good mechanicalproperties.

The fluorination treatment can also be eflected upon a compressedmaterial obtained from the powder while subjecting the material toplastic deformation, for example rolling or drawing.

A fluorination agent can also be reacted with a more or less compactmass formed previously by compression of the powder. The mass can beproduced by compression in the cold or at an elevated temperature.Either a non-fluorinated compacted powder or one previously compressedcan be treated in the process.

Irrespective of the procedure adopted, a thin coating of magnesiumfluoride is obtained on the metal particles, having a thickness of aboutone hundredth of a micron. The combined fluorine content of the metalshould be controlled so as to have a value from 0.1 to 15% by weight inthe final product. For a given sample of powder, the fluorine contentwhich allows given mechanical properties to be obtained is a function ofthe particle size of the powder. With fine powders, having a particlesize below 60 microns, the combined fluorine content preferably liesbetween 0.5 and 2%, if good ductiltiy with flow resistance are to beobtained.

In general, as the particle size of the powder increases the value ofthe rupture load decreases, while the rupture elongation valueincreases. For a powder of given particle size, increase in the fluorinecontent has the eflect of very slightly increasing the rupture loadvalue and greatly decreasing the rupture elongation value. Use of theprocess of the invention consequently allows predetermined mechanicalproperties to be obtained, by varying the particle size of the powder orthe fluorine content or both.

Compaction of the fluorinated powders is preferably carried out inseveral stages:

Table Particle size range of magnesium 10-65 10-65 10-65 5-40 Compositepowder (microns). Mg-MgO material, 100-200 Specific surface (mi/g.) 0.25 0.25 0. 25 0. 50

Typo of treatment Hydrogen Gaseous Gaseous Gaseous No fiuorinatluoride,1 fluorine, 2 fluorine, 2 fluorine, 2 tion hr. at 290 hrs. at 330 hrs.at 475 hrs. at 475 treatment 0. C. O. C.

Fluorine content as percent by weight of product obtained 0.88 2 6 12Thickness of fluorine on each grain (mircons) 0.02 0.05 0. 14 0. 14

Average mechanical properties at 20 C.

R, kgJmm. 33. 7 32. 3 34. 8 35 28 E, kg./mm. 31. 4 30. 6 33.1 34. 21 Apercent (elongation) [V678 4. 5 3. 6 2. 01 1. 4 3 A (Brinell hardness)57. 5 53. 4 59 69 47 Average mechanical properties at; 450 C.

R, k.g/mm. 1.65 2.03 2.6 2. 8 1. 2 K, kg./mm. 1. 4 1. 75 2. 2 2. 5 1 Apercent (elongation)/; 67S 13. 5 13. 9 10. 5 4. 3 4

Firstly, compression is effected in the cold with a compressive force of4 to 60 metric tons/mm With low compressions, it is advantageous firstto place the powder within an auxiliary container of magnesium.

One or more compressions in the hot are then eiiected, depending uponthe particle size range of the powder used, at temperatures below 600C.If the powders have particle sizes below 100 microns, two or threesuccessive compressions at increasing temperatures are of advantages,since if compression at the maximum temperature were effectedimmediately after cold compression, oxidation would occur or evenignition of the magnesium powder.

The compressed materials so obtained can then be subjected to plasticdeformation by the standard techby heating in air.

All the powder samples were subjected, after fluorination to acompaction treatment effected in a drawing mill having a drawing dieratio, S/s, of 49, with a compressive force of 40 metric tons/mm. Thefour following stages were employed.

' (a) Compression in the cold;

(b) Compression at 350C.; (0) Compression at 450C.;

(d) Compression at 500C.

By way of comparison, the right-hand column of the table shows theresults obtained for a material prepared by the above compactiontreatment, starting with a non- ;fiuorinated composite Mg-MgO material,having a particle size range of 100-200 microns. The material obtainedwas taken up to 450C. by heating in air. It may be mentioned in thisconnection that, if a very fine Mg-MgO powder is used, having a particlesize range of 10 to 65 The table shows the improvement in mechanicalproperties which can be obtained by using the process of the invent-ion.The rupture load, elasticity and rupture elongation values, both coldand hot, are clearly better than the corresponding values of knownmagnesium-base materials, such as magnesium alloys or compositematerials of the Mg-MgO type.

Apart from the mechanical properties described above, the fluorinatedand compacted materials of the invention are of great interest becauseof their remarkable resistance to corrosion in air. They can be heatedto 500 C. in air saturated with water vapour Without appreciablycorroding, the increase in weight being less than 1 mg./ cm. afterexposure for 800 hours, whereas with ordinary magnesium-base materials avery large degradation occurs in air at 400 C. and, from 350 C.,oxidation is very substantial and can have serious effects.

The ignition temperature in humid air of the novel material is 640 C.and is about 40 to 50 C. higher than that of pure magnesium andundergoes much less violent combustion.

The process of the invention thus yields a novel material which is ofparticular interest in use in view of the following advantages:

(a) Mechanical properties in the cold and, particularly, in the hot,which are greatly superior to those of ordinary magnesium-basematerials;

(b) Possibility of obtaining very varied mechanical properties asrequired, by varying either or both of the particle size range and theshape of the particles of the initial powder or the combined fluorinecontent, particularly by varying the temperature and/or duration of thetreatment and the composition of the gaseous phase used;

(c) Resistance to oxidation by air, even humid, which is greatlyimproved at elevated temperatures, permitting use of magnesium up to atemperature of about 500 C., whereas operation was previously limited tobelow about Such characteristics have never previously been obtained andmake the new material particularly suitable for use as a claddingmaterial for fuel elements in nuclear reactors operating at hightemperatures.

What is claimed is:

1. A process for the preparation of an improved metall'ic materialformed from the compression of a member selected from the groupconsisting of a magnesium powder and a magnesium alloy powder comprisingthe steps of subjecting particles of said powder having a size range offrom 1 to 2,000 microns to a gaseous atmosphere consisting essentiallyof a fluorine containing gas at a temperature from 0 C. to 600 C. untila material containing 0.1 to 15% by weight of combined fluorine as athin film of magnesium fluoride on the particles is produced andcompressing said particles of powder.

2. A process according to claim 1, in which the atmosphere contains thefluorination agent and an inert gas.

3. A process according to claim 1, in which the fluorination agent isfluorine.

4. A process according to claim 1, in which the fluorination agent ishydrogen fluoride.

5. A process according to claim 1, in which the powder is firstsubjected to the action of the fluorination atmosphere for a timesufficient to attain the desired degree of fluorination of themetal-containing powder and is then compressed.

6. A process according to claim 1, in which heating is effected for aperiod of 2 to 5 hours.

7. A magnesium-base material comprising a powder, the particles of whichcomprise substantially magnesium,

have a size range from 1 to 2,000 microns and are covered with a thincoating of magnesium fluoride, the combined fluorine content of thecoated metal particles being between 0.1 to 15% by weight.

References Cited by the Examiner OTHER REFERENCES Metals Handbook,volume 1, American Society for Metals, Metals Park, Ohio, 1961. TA 472A3, pages 1095-1111.

LEON D. ROSDOL, Primary Examiner.

RAY K. WINDHAM, CARL D. QUARFORTH,

Examiners.

A. I. MUCCINO, R. L. GOLDBERG, R. L. GRUD- ZIECKI, Assistant Examiners.

1. A PROCESS FOR THE PREPARATION OF AN IMPROVED METALLIC MATERIAL FORMEDFROM THE COMPRESSION OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF AMAGNESIUM POWDER AND A MAGNESIUM ALLOY POWDER COMPRISING THE STEPS OFSUBJECTING PARTICLES OF SAID POWDER HAVING A SIZE RANGE OF FROM 1 TO2,000 MICRONS TO A GASEOUS ATMOSPHERE CONSISTING ESSENTIALLY OF AFLUORINE CONATINING GAS AT A TEMPERATURE FROM 0* C. TO 600*C. UNTIL AMATERIAL CONTAINING 0.1 TO 15% BY WEIGHT OF COMBINED FLUORINE AS A THINFILM OF MAGNESIUM FLUORIDE ON THE PARTICLES IS PRODUCED AND COMPRESSINGSAID PARTICLES OF POWDER.